Windows - Proxy settings for all users

What the GPO does is setting the value ProxySettingsPerUser of the registry key HKLM:\Software\Policies\Microsoft\Windows\CurrentVersion\Internet Settings. Enabled is 0, disabled is 1.

Reference: Make proxy settings per-machine (rather than per-user)

Key: HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Internet Settings\  
Value: ProxyServer

Key: HKLM\SOFTWARE\Wow6432Node\Microsoft\Windows\CurrentVersion\Internet Settings\  
Value: ProxyServer

String: [HostnameOrIP:Port]

Also see:

https://blogs.msdn.microsoft.com/askie/2017/06/20/what-is-defaultconnectionsettings-key/

Sorry to revive an old thread, but I figured this might help anyone looking for this answer. This stumped us for a couple days as well, but one of the answers pointed us in the right direction. The settings are stored as Binary in:

HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Internet Settings\Connections

DefaultConnectionSettings 
SavedLegacySettings

To get the binary we needed I made the changes required and then did an export of the same settings under HKCU, edited the .reg file to point to HKLM, and then imported.

Hope this helps someone!